It is possible to obtain a significant reduction in the NOx emitted by a diesel engine while in operation by post-treatment of the exhaust gas using a catalyst of the NOx trap type.
Such a catalyst operates on the principle of enabling NOx to be stored in the catalyst by forming a stable complex of the Ba(NO3) 2 type.
This storage of NOx then takes place while the engine is operating normally on a lean mixture, i.e. with excess oxygen.
Once the catalyst is saturated in NOx, it becomes necessary to purge it so as to reduce the amount of NOx stored in the trap.
This can be done, for example, by switching the operation of the engine from a lean mode to a rich mode, i.e. a mode with excess fuel.
Storage efficiency decreases as the trap becomes filled with NOx. Consequently, it is possible to obtain NOx conversion that differs depending on the spacing between purges and on the duration of such purges.
Similarly, the quantity of NOx that can be stored in a trap is not constant, since it depends on the temperature of the catalyst, and thus on the temperature of the exhaust gas from the engine.
Thus, for example, a single lean/rich operating sequence, e.g. 100 seconds (s) with a lean mixture and 5 s with a rich mixture, leads to conversion that differs depending on the temperature at which the sequence is performed.
Finally, the engine can be operated in rich mode (e.g. in order to reduce NOx) only under certain running conditions of the vehicle.
Consequently, optimizing NOx conversion in a NOx trap needs to take account of various parameters relating to the operation of the engine, e.g. to physical magnitudes, etc. . . . Deciding when to trigger and when to stop purges is a key factor governing the performance of the post-treatment system, i.e. the NOx conversion that is achieved, the associated extra consumption of fuel, the penalty in terms of other pollution emitted, . . . .
Applying a NOx trap to a diesel-engined vehicle thus requires strategies that enable purges to be managed autonomously. These strategies must be defined in such a manner as to optimize the various services expected of the system.
In practice, these strategies are implanted in a computer, e.g. an engine control computer, and they are intended to control the operation of the catalyst in interaction with the other strategies involved in controlling the engine.
Various methods and systems have already been envisaged for optimizing such purges.